The present invention relates to a storage unit and, particularly, to a technique effectively applied to a feeding technique for a RAID (Redundant Array of Inexpensive Disk) system in which storage devices are arrayed so as to have redundancy.
According to the examination by the inventor of this invention, the techniques as follows are known in the conventional RAID system and the feeding technique for the RAID system.
The conventional RAID system examined by the present inventor comprises: a hard disk drive for storing data; a disk adaptor for controlling the writing/reading of data to/from the hard disk drive; a channel adaptor for controlling an interface with a host CPU; a cache memory for temporarily storing the data written/read between the host CPU and the hard disk drive; a cache switch for shifting the writing/reading of data to the cache memory; a shared memory for storing configuration information of the entire unit and information for managing the progress of operation; an AC-DC power source, a DC-DC power source, and a battery for a feeding system; and the like.
In this conventional RAID system, the writing data from the host CPU is once written to the cache memory and then stored in the hard disk drive. At this time, it is unnecessary to immediately write, to the hard disk drive, the data stored in the cache memory, and the writing may be done depending on operational conditions of the RAID system. Even if the data writing from the cache memory to the hard disk drive is not done, the data temporarily stored in the cache memory from the host CPU may be accessed. Furthermore, there is an advantage of the fact that since an access speed of the cache memory is faster than that of the hard disk drive, a high-speed data access can be done from the host CPU. In the RAID system, the cache memory with larger capacity is more preferable from the viewpoint of the data access performance.
Therefore, the conventional RAID system uses a large number of DRAMs (Dynamic Random Access Memories) because the DRAM has the large memory capacity and is effective in cost. The DRAM used for the cache memory is inferior to a SRAM (Static Random Access Memory) in the access speed, but is advantageous in memory capacity and the cost. Also, the DRAM is advantageous over the flash memory in both of the cost and storage capacity. In contrast, since the DRAM is a volatile memory, it is necessary to always supply a power source voltage in order to store (retain) the data and also a refresh operation (regular recharging operation for the inner capacitor) unique to the DRAM is indispensable.
As described above, since the DRAM is a volatile memory, when power failures such as blackouts occur under the condition that the data in the cache memory is not yet stored in the hard disk drive, the data in the cache memory (data not yet written in the hard disk drive) is volatilized (data lost) due to the stop of the feeding to the cache memory. Therefore, the function to prevent the volatilization of the data in the cache memory, for example, the function to switch the power source to the battery power source and continue the feeding to the cache memory during the blackout is needed in the power source feeding system of the RAID.
For example, from the viewpoint of the reliability of the stored data, it is essentially desirable that the system is stopped after the data not yet written to the hard disk drive in the cache memory is stored into the hard disk drive even when the power failures such as the blackouts occur. However, a high-cost power system such as UPS (Uninterruptable Power Supply) is required for its achievement.
Therefore, in the conventional system, a battery backup system for the cache memory data is adopted as a method of preventing data loss at low cost. In this battery backup system, the battery is used only for the feeding to the cache memory during the blackout failures, and once the cache memory (DRAM) is switched to the backup operation mode, the data access is inhibited and the power consumption thereof is reduced. Accordingly, the low-cost lead storage battery can sufficiently cover the battery performance and the power capacity. Also, the system may be used even in a mounting space capable of mounting on a device.
As described above, the feeding system in the conventional RAID system examined by the inventor uses, for example, a feeding path as described in a document of Japanese Patent Laid-Open No. 2001-290608. More specifically, the document discloses a technique provided with two systems such as a normal feeding system in which the AC voltage of commercial power source is converted to the DC voltage and the converted DC voltage is converted to a voltage necessary for the normal operation of the cache memory and the converted voltage is supplied to the cache memory, and a battery feeding system, employed at the time of occurrence of the power failures such as the blackouts, in which a voltage of the battery is converted to the minimum voltage necessary for the backup operation of the cache memory and the converted voltage is supplied to the cache memory.
As a result of the examination by the inventor about the feeding system of the conventional RAID system, the followings have been shown.
For example, the feeding system in the conventional RAID system examined by the inventor has a configuration as shown in FIG. 9. An example of the configuration and operation of the feeding system in the conventional RAID system examined by the inventor will be described with reference to FIG. 9.
As shown in FIG. 9, a feeding system 61 in the conventional RAID system examined by the inventor is connected to a host CPU 2 and a commercial power source 3 and is composed of: a cache memory (CACHE) 62; a hard disk drive (HDD) 11; a disk adaptor (DKA) 12; a channel adaptor (CHA) 13; a cache switch (CSW) 15; AC-DC power sources (1) 63 to (4) 66; a battery 67; DC-DC power sources (1) 68 and (2) 69; and the like.
In the configuration of this feeding system 61, the path from the AC-DC power source (2) 64 through the switch (SW1) to the memory 71 and the path from the battery 67 through the DC-DC power source (2) 69 and the switch (SW2) to the memory 71 are provided. The former is the feeding system for the normal operation mode and the latter is the feeding system for the backup operation mode. In addition, the power from the AC-DC power source (1) 63 is supplied to the cache memory 62, the disk adaptor 12, the channel adaptor 13, and the cache switch 15, and the power from the AC-DC power source (4) 66 is supplied to the hard disk drive 11. Note that the battery 67 is always charged through the AC-DC power source (3) 65 and the DC-DC power source (1) 68.
In the above-described feeding system 61, the AC-DC power sources (1) 63 to (4) 66 are usually used to supply the power to the whole RAID system, and the AC-DC power source (2) 64 is usually used to supply the power to the cache memory 62. In this case, the switch (SW1) is in a turn-on state. When the blackout occurs in this state, the AC-DC power sources (1) 63 to (4) 66 are stopped. However, a power source switching control circuit 72 turns on the switch (SW2) simultaneously with the detection of the reduction in output voltages of the AC-DC power source (2) 64, and the power is temporarily supplied from both of the battery 67 (DC-DC power source (2) 69) and the AC-DC power source (2) 64. Then, the switch (SW1) is turned off and finally the feeding path is completely switched to that from the battery 67. Therefore, the feeding to the cache memory 62 is not stopped, and loss of the data not yet written to the hard disk drive 11 is not generated.
However, since the operation voltage of the DRAM of the memory 71 used in the cache memory 62 is, for example, 3.3 V±10% in the above-described feeding system 61, the switching of the feeding path is started when the output voltage of the AC-DC power source (2) 64 is reduced to, for example, 3.3 V−7%. In this switching of the feeding path, it is necessary that the mode of the cache memory 62 is changed to the backup operation mode to reduce current consumption to a level adequate to the feeding ability of the battery 67 in a short time when the feeding voltage is reduced from −7% to −10% (μS order). Therefore, in the conventional feeding system 61, it is necessary to adopt a highly sensitive circuit configuration, as a series of control circuits used to monitor the feeding voltage and switch the feeding path, and thus the mounting technique to cope with such an external factor as noise is indispensable.
Also, in the conventional feeding system 61, when the instantaneous blackout beyond the endurance of the AC-DC power sources (1) 63 to (4) 66 occurs, the cache memory 62 is inevitably turned into the backup operation mode (in the state where the cache memory 62 cannot be used). Therefore, the operation of the device is substantially stopped though the data loss is not generated. For its recovery, the assist of engineers is necessary. As described above, the feeding system 61 in the conventional RAID system leaves room to be improved in the endurance for blackout.
The problems as shown above can be solved by the improvement of the endurance for instantaneous blackout of the feeding system, and if the endurance for instantaneous blackout can be improved, the data in the cache memory can be stored also into the hard disk drive (destaging process) even at the time of the blackout or the instantaneous blackout. As a result, it becomes possible to establish the RAID system with higher reliability. However, it is thought that the data backup of the cache memory is still necessary even if the endurance for blackout is improved and a destaging process thereof can be performed. This is because there is the possibility of exceeding the endurance limit for blackout when the data storage process is repeatedly performed due to the failure of the hard disk drive during the is destaging process. Therefore, it is necessary to switch the destaging process to the data backup process for the data in the cache memory in such a case.